1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor with shock resistance, and a method of manufacturing the same.
2. Description of the Related Art
A conventional semiconductor acceleration sensor is shown in FIG. 15A and FIG. 15B. This sensor has: a frame 101 having an opening inside thereof; flexible beams 102 extending from the frame 101 to the inside of the opening of the frame 101 and having an intersection portion; a weight 103 suspended from and supported by the intersection portion of the beams 102 so that the weight can freely move; piezoresistors (not shown) which are mounted on the beams 102 and which vary the resistance values with distortions generated in the beams 102 due to displacement of the weight; and a seating 105. (Refer to e.g. Japanese Laid-open Patent Publication Hei 8-327656 and Japanese Laid-open Patent Publication Hei 11-135804.)
The frame 101 is formed of e.g. a semiconductor substrate made of silicon, and has a roughly rectangular shape, as seen in plan view, with an opening 106 inside thereof. The beams 102 are provided each to extend inward from an upper surface of each of the four sides constituting the frame 101, and intersect with each other at around the center of the inside of the frame 101 to form an intersection portion 102a. Each of the beams 102 is formed to be thin so as to be flexible.
Plural piezoresistors (not shown) are provided on the surfaces of the beams 102, and are respectively provided in the vicinity of the intersection portion 102a and at four base end portions 102b of the beams 102. Assume here that a direction being parallel to any arbitrary one of the sides of the frame 101 is X-axis, that a direction being parallel to another side of the frame 101 and intersecting the X-axis at an angle of 90 degrees is Y-axis, and that a direction intersecting each of the X-axis and the Y-axis at an angle of 90 degrees is Z-axis. Under this assumption, three sets each of four piezoresistors corresponding to the X-axis, the Y-axis and the Z-axis form three Wheatstone bridges to detect accelerations working in the directions of the three axes, respectively. These piezoresistors are further connected to electrodes (not shown) formed on the frame 101.
The weight 103 is connected to and suspended from a lower surface of the intersection portion 102a of the beams 102, whereby the weight 103 is supported by the frame 101 so as to be freely movable owing to the flexibility of the beams 102. The weight 103 has a cross section of trapezoidal shape having a width decreasing in the direction of the Z-axis from the frame 101 to the seating 105. Furthermore, the weight 103 has a thickness smaller than that of the frame 101 in order to provide a space for the weight 103 to be able to freely move in response to acceleration working thereon. At corner portions of the opening 106 of the frame 101, stoppers 104 are provided to limit the displacement of the weight 103. Each stopper 104 has a rough shape of triangle, two sides of which are supported by and along the corresponding sides of the frame 101. The stoppers 104 and the seating 105 have functions to limit amount of free movement of the weight 103. As seen in plan view, the seating 105 has a shape of rectangle roughly the same as the outer shape of the frame 101.
When acceleration works on the semiconductor acceleration sensor having the above-described configuration, the weight 103 moves back-and-forth longitudinally, laterally and/or vertically according to the direction and magnitude of the acceleration working thereon, because the weight 103 is supported by the frame 101 and the beams 102 so as to be freely movable. At this time, deflections occur in the beams 102 so that plural ones of the piezoresistors are subjected to stresses, whereby their resistances vary. As a result, the Wheatstone bridges lose the equilibrium, whereby electrical signals corresponding to the acceleration working thereon are output from the Wheatstone bridges corresponding to the respective axes. By taking the electrical signals from the electrodes, the acceleration can be detected. When excessive acceleration works on the beams 102, the stoppers 104 and the seating 105 limit the free movement of the weight 103, thereby preventing them from being broken.
However, according to such semiconductor acceleration sensor, there is a possibility that shocks may be locally concentrated when the weight 103 impacts the stopper(s) 104. Because of its influence, it has been difficult to improve the stoppers 104 with respect to resistance to breaking.